1. Field of the Invention
This invention relates to a rigid internal gear of a wave gear drive, and in particular to a rigid internal gear which is lightweight without deteriorating strength, wear resistance and the like thereof.
2. Related Art Description
A typical wave gear drive has a circular rigid internal gear, a cup-shaped flexible external gear disposed inside the rigid internal gear, and a wave generator fixedly inserted into the flexible external gear. The wave generator is generally shaped to have an elliptical profile, which deforms the flexible external gear into an elliptical shape so as to engage external teeth partially with internal teeth of the rigid internal gear. Rotation of the wave generator by a motor or the like causes to move the engaging portions of the teeth in the circumferential direction. Since there is a difference in number of teeth between the external and internal gears, relative rotation between the both gears occurs according to the difference in number of teeth. Thus, by fixing one of the gears, reduced rotational output can be obtained from the other gear.
The wave gear drive is characterized in that it has a small number of component parts and is lightweight in comparison with reduction mechanisms of other types. In order to realize more lightweight wave gear drive, the rigid internal gear as one of component parts thereof must be lightweight. More specifically, since the rigid internal gear requires a desired strength and wear resistance, it is typically made of ferrous or copper material. This means that the rigid internal gear occupies the most part of weight of the wave gear drive. In particular, where the rigid internal gear is formed integrally with a flange or housing for mounting the wave gear drive on the side of a motor or an output-side member, the rigid internal gear is extremely heavy. This type of rigid internal gear becomes an obstacle to achieving the lightweight of the wave gear drive, which is not preferable.